1. Field of the Invention
The invention relates to a parallel-detector electron energy-loss spectrometer including a device for dispersing electrons according to their energy such as a magnetic sector, a parallel electron detector consisting of several detection elements, and several quadrupole electron lenses.
2. Description of Prior Art
Modern electron microscopes are capable of imaging individual atoms in a thin sample, but the images produced by the microscope alone contain no direct information on the chemical composition of the sample. The composition can be determined by producing and analysing the spectrum of characteristic energy losses experienced by the electron beam as it traverses the sample. Consequently, an electron energy-loss spectrometer is a widely used attachment to an electron microscope.
Since the electron beam gradually erodes the thin sample by a process known as radiation damage, an important consideration for an electron energy-loss spectrometer is the efficiency with which it can detect the energy-loss spectra. An efficient method for detecting the spectra is to employ a detector consisting of several detection elements which operate simultaneously. Devices which can be used in this role include photographic plate or film directly exposed to the electron beam, or a scintillator which converts the electrons into a light image which is in turn detected by a TV camera, a photodiode array, or a charge-coupled device array. The devices are often described as parallel detectors, and electron energy-loss spectrometers employing parallel detectors are known as parallel-detection electron energy-loss spectrometers.
The parallel detectors require that the magnification (dispersion) of the spectra be larger than can be reasonably produced by an energy-dispersing device such a magnetic sector, and that the magnification of the spectra be variable in the energy-dispersion direction. In prior art parallel-detection electron energy-loss spectrometers, round electron lenses were used to increase and vary the magnification of the spectra. A major disadvantage of such prior art devices was that they were unable to vary independently the magnification of the spectrum in the dispersion direction, and the width of the spectrum in the direction perpendicular to the dispersion direction. Accordingly, when these devices were adjusted to produce large energy dispersion, the width of the energy spectrum also became large, and a part of the spectrum fell outside the active area of the detector. Useful electron signal was therefore lost, which decreased the efficiency of the analysis. Conversely, at small energy dispersions the width of the spectrum became small, and the electron beam was concentrated on a small area of the detector. This resulted in an unnecessarily high intensity of the electron beam per unit area of the detector. The high electron intensity typically produced a high rate of radiation damage in the detector, and shortened the useful life of the detector. Futher, round lenses rotated the spectra by an angle which varied with the magnification of the spectra. This complicated the mechanical construction of the spectrometer, because the detector had to be rotatable so as to remain aligned with the energy-dispersion direction.